Aluminum composite

ABSTRACT

An aluminum composite having improved penetration and spall resistances comprising a high-strength aluminum frontal member having a tensile strength between 65,000 and 90,000 p.s.i. and an aluminum backing member having a tensile strength between about 45,000 and 70,000 p.s.i. The backing member must have a thickness between about 4 and 20 percent of the total composite thickness and have a tensile strength of at least 2,000 p.s.i., preferably 7,000 p.s.i., less than the frontal member.

XR 3,649,227 SR Fetzer et al.

[54] ALUMINUM COMPOSITE [72] Inventors: Maurice C. Fetzer, Walnut Creek;John D.

Sprowl, Pleasanton; William R. Mohondro, Dublin, all of Calif.

[73] Assignee: Kaiser Aluminum & Chemical Corporation, Oakland, Calif.

[22] Filed: Jan. 26, 1970 [21] App1.No.: 5,860

[52] US. Cl ..29/l97.5, 29/1835, 148/34 [51] Int. Cl ..B32b 15/00 [58]Field ofSearch.... ..29/197.5

[56] References Cited UNITED STATES PATENTS 2,821,014 4/1935 Miller..29/197.5

[451 Mar. 14, 1972 Brown ..29/197.5

Gauthier ..29/197.5

Primary Examiner-,Hyland Bizot Attorney-James E. Toomey, Paul E. Calrow,Harold L. Jenkins and Edward .I. Lynch [57] ABSTRACT 6 Claims, 1 DrawingFigure -BAC/(//V6 MEMBER 74/00/555 0/ form 7 //(K/1/55-" ALUMINUMCOMPOSITE BACKGROUND OF THE INVENTION The demand for high spall andpenetration resistance lightweight materials has increased considerablyin the last few years, particularly for armor plate applications. To acertain extent, the 7039 alloy (Aluminum Association designation) hasenjoyed much popularity due to the low spalling and high-penetrationresistance to projectiles characteristic of this alloy. In addition, thealloy has high strength and corrosion resistance as well as being easilywelded by conventional techniques.

With aluminum materials, the resistance to armor piercing (AP)projectiles is directly related to the tensile strength and hardness ofthe material. However, as the strength and hardness increase, thetendency to spall from projectile impact, such as fragment simulator(FS) projectiles, increases. With aluminum materials having tensilestrengths above about 65,000 to 70,000 p.s.i., the spalling from F8projectiles is so great that the materials are more or less useless asarmor plate. H. P. George et al., in US. Pat. No. 3,042,555 attempted toovercome this problem by overaging the rear face of 7075 and 7178 alloyplates. However, the process described is very difficult to control,i.e., it is difficult to obtain consistent properties, and, although theproduct exhibits improved spalling resistance, the improved propertiesare obtained at the expense of a reduced AP resistance.

For aluminous materials, the 7039 alloy provides the best balance ofpenetration and spalling resistance. However, for many applications the7039 alloy is not completely satisfactory due to the relativelylow-ballistic resistance to armor piercing (AP) projectiles striking theplate at obliquities greater than 30, i.e., a projectile path at anangle greater than 30 from a line normal to the surface. At anglesgreater than 30 high-hardness steel armor has superior ballisticresistance on an equal weight basis.

SUMMARY OF THE INVENTION This invention relates to an all-aluminumcomposite materi- 21 having improved penetration resistance andresistance to spalling. The present invention provides a compositematerial having improved resistance to penetration by armor piercing(AP) projectiles while maintaining, and in many instances increasing,the resistance to impact by other projectiles such as a fragmentsimulator (FS) projectiles.

Generally, the composite of the present invention comprises a frontalmember of a high strength aluminum alloy having a tensile strengthbetween about 65,000 and 90,000 p.s.i. and a tough aluminum backingmember having a tensile strength between about 45,000 and 70,000 p.s.i.The thickness of the backing member should be between 4 and percent ofthe total composite thickness. Within the context of this invention, thetotal thickness of the composite ranges from onefourth to about 6 inchesthick. The tensile strength of the frontal plate should exceed thetensile strength of the backing member or cladding by at least 2,000p.s.i., preferably greater than 7,000 p.s.i. The various members mustexhibit the above strength values in the composite material.

A particularly advantageous aluminum alloy for the frontal plateconsists essentially of 4 to 7 percent zinc, 1.5 to 5 percent magnesium,up to 3.0 percent copper, and the balance aluminum and inconsequentialamounts of other elements, i.e., amounts which do not detrimentallyaffect the penetration and spalling resistance in a substantial manner.

DETAILED DESCRIPTION OF THE INVENTION percent, preferably between about5 and 12 percent of the thickness of the composite. An aluminum alloy isherein considered to be an aluminum base alloy having more than 80percent aluminum.

Many aluminum alloys, such as 7075T6, 7079-T6, 7178T6, 2024-T3 and thelike, have been found to have the desired characteristics for thefrontal member within the broadest scope of the present invention. Theprimary requirement for the frontal plate composition is that it must becapable of developing between about 65,000 and 90,000 p.s.i. tensilestrength upon appropriate heat treatments after bonding with the backingmember. The higher strength alloys, such as 7178T6, have a greatertendency to shatter when struck by higher caliber projectiles such as0.50 cal and 20 mm. projectiles; and this characteristic severely limitsthe multihit capabilities of this material. However, such high-strengthalloys employed in the composite of the present invention do exhibitsuperior single hit properties. The preferred alloy composition of thefrontal member consists essentially of about 4 to 7 percent zinc, 1.5 to5 percent magnesium, up to 3.0 percent copper, and the balance aluminumand inconsequential amounts of other elements which do not substantiallyaffect the strength and ballistic properties of the alloy. Generally, ahigh magnesium to zinc ratio provides a member which is more easilywelded by conventional GMA techniques.

The backing member or cladding comprises a relatively medium strength,tough aluminum alloy having a tensile strength between about 45,000 and70,000 p.s.i. The backing member tensile strength must be at least2,000, preferably 7,000 p.s.i., less than the tensile strength of thefrontal plate. For substantially improved penetration resistance andspalling resistance, the backing member must be between about 4 and 20percent, preferably between about 5 and 12 percent, of the totalcomposite thickness. Any heat treatable aluminum alloy can be employedif the backing member exhibits the required strength in the finalcomposite form. The preferred alloy for the backing member consistsessentially of 4 to 6 percent zinc, 0.5 to 3.5 percent, preferably 0.5to 1.5 percent, magnesium and the balance aluminum and inconsequentialamounts of other elements. To facilitate the roll bonding of thecomposite, the weight ratio of the magnesium content of the backingmember to themagnesium content of the frontal member should be less thanabout 0.7. Other suitable alloys include 7039 and 6053.

In the alloys-described above for the frontal and backing members thesilicon should not exceed 0.3 percent, iron 0.4 percent, manganese 0.5percent, titanium 0.10 percent, chromium and/0r zirconium 0.25 percent.Although the above limitations are primarily given as maximum amounts ofimpurities, positive additions of these elements are included within thescope of the invention. For example, chromium and zirconium may be addedor controlled in the composition between 0.10 and 0.20 percent toimprove stress corrosion resistance and inhibit recrystallization;copper may be added or controlled between 0.10 and 0.20 percent toimprove stress corrosion resistance in the T6 temper; manganese may beadded or controlled between 0.1 and 0.5 percent to improve strength andstress corrosion resistance; and titanium may be added or controlledbetween 0.01 and 0.03 percent for grain refining purposes. Generally,for most commercial applications the copper content will be less than0.2 percent for welding purposes although it is recognized that alloyshaving between 2 and 3 percent copper can be welded by conventionaltechniques.

All compositions given herein are on a weight percent basis and allstrength properties are in the long-transverse direction. The expressionlong-transverse direction" refers to the direction normal to thedirection of metal working or flow and in the longer dimension of across section taken in a plane normal to the direction of metal flow.

It has been found that the composite material of the present inventionexhibits substantially improved penetration resistance to AP projectileswith substantially no reduction in spalling resistance in comparison to7039-T64. In most of the composites tested, a slight increase inspalling resistance was noted in comparison with 7039-T64 on an equalweight basis. The improvement in spalling resistance would be consideredsubstantial if compared with materials having equivalent AP resistancesuch as high-hardness homogeneous steel plate and high-strength aluminumalloy plate such as 7075-T6 and 7178T6.

A critical feature of the instant discovery is the relationship of thethickness of the backing member with respect to the total thickness ofthe composite to obtain an improved spalling resistance, and attendantwith the improved spalling resistance, a substantially improvedresistance to penetration. It has been found that a backing memberthickness between about 4 and 12 percent provides for improvedresistance to AP projectiles. Greater or lesser amounts than thisgenerally provides for no substantial improvement over the frontal platealone. A maximum resistance to AP projectiles is reached with a backingmember thickness of about 5 to 6 percent, which is particularlynoticeable when the projectiles strike at high obliquities, e.g., 45.However, the resistance to F8 projectiles is more or less a function ofthe backing member thickness up to a thickness of over 20 percent of thetotal composite thickness. The greater the thickness of the backingmember, the greater is the resistance to ES projectiles. A backingmember thickness between about 4 percent and 20 percent, preferablybetween about 5 percent and 12 percent, provides the best balance of APand FS penetration resistance. The relationship of backing memberthickness to ballistic properties for the preferred alloys of thepresent invention is indicated inthe drawing. The nominal compositionfor the frontal member was 6.6 percent zinc and 2.5 percent magnesiumand for the backing member was 5.5 percent zinc and 0.75 magnesium. Thepresent invention uniquely allows the ballistic properties of thecomposite to be specifically designed to satisfy the requirements of awide range of applications.

The several components of the present invention are joined to form thecomposite through an intimate metal-to-metal bond, which can be formedby several methods, such as roll bonding, explosive bonding and thelike. It is believed that the composite structure acts as an energyabsorber and disperser, in that, as the projectile penetrates thefrontal plate, the plug displaced thereby forces the backing member tobe stretched and torn away from the frontal plate.

Conventional roll-bonding techniques are preferred which include heatingthe members to a temperature between about 650 to 900 F., passing themembers through rolls several times with slight thickness reduction toseal the backing member to the core, then subsequently furtherhot-rolling the members to form the desired composite. The thicknessreduction of the composite in the hot-rolling step should normallyexceed 50 percent, preferably about 80 percent.

With aluminum alloys containing more than about 1.5 percent magnesium, asound bond is difficult to obtain by conventional roll-bondingprocedures; and it has been found that a more complete bond can be madeby precladding these alloys with small amounts (e.g., one-half to 2percent of the total thickness) of relatively magnesium-free aluminumalloy, i.e., not more than 0.1 percent magnesium, then roll bonding thepreclad members with the precladded faces in contact. Suitableprecladding alloys include 3003, 1 100, 7072 and 1230.

Due to the fact that the elements are bonded to form the composite, thealloy selection is restricted because both plates must usually respondto the same thermal treatment to reach the required strength properties.The preferred alloy compositions of the present invention, set forthabove, are particularly compatible in this regard. Many artificial agingtreatments can be employed with the preferred alloy composition of thepresent invention. For suitable ballistic properties, the solution heattreated composite may be heated to between 220 and 250 F. and maintainedat that temperature between about 24 and 48 hours, depending onthickness of the composite and aging temperature. Other aging practicescan of 5 course be employed. It has been found that overaging, by eithersingle-step or multiple-step aging practices, increases the spallingresistance of the exposed face of the frontal plate, particularly atprojectile paths greater than 30 from a line normal to the surface.

Several examples of the present invention are given in Table l. Acomparison of ballistic properties of the examples with 7039-T64 isgiven in Table 11. The specimens of the examples were prepared by firstprecladding the frontal member and backing member on one face with 1,230alloy in thickness between 1 and 2 percent, heating the members to aroll-bonding temperature between 650 and 850 F. and then roll bondingthe members (with the 1230 alloy surfaces in contact) to effectthickness reductions between 60 and 80 percent. The specimens ofExamples 1 through 9 were solution heat treated by heating at 850 F. andquenching in cold water, and subsequently aged to the T-6 temper. Theartificial aging practice consisted of naturally aging for 5 days,heating at a rate of 35 F./hr. to the aging temperature of 240 F. andartificially aging at that temperature for 48 hours. The specimen forExample 10 was given conventional thermal treatment to obtain the T-6temper. The tensile properties (long transverse direction) of themembers of the samples are shown in Table l.

The prepared samples were then subjected to ballistic tests with Cal.0.30 AP projectiles at 0 obliquity, Cal. 0.50 AP projectiles at 45 and 0and mm. AP projectiles at 30 and 20 mm. FS as shown in Table 11. Thetests comprised determining the lowest velocity for complete penetrationby the projectile and the maximum velocity for partial penetration bythe projectile. The ballistic limit is the average of these twovelocities. The ballistic limits were compared with known ballisticlimit values for 7039-1'64 to determine the thickness of this alloynecessary to give the same protection, i.e., same ballistic limit as thecomposite material, and then the merit rating was determined accordingto the formula given in Table ll.

As is evident from the merit ratings shown in Table II, considerableincreases in AP resistance can be obtained with the composite materialof the present invention. More importantly, no substantial reduction inPS resistance was found and in many cases, as in Examples 4 and 5, anincrease in PS resistance was noted. Although the improvement in PSresistance may seem inconsequential, the improvement is considerablewhen compared with other materials having equivalent AP resistance.Moreover, the invention provides for up to a percent decrease in weightrequirements over 7039-T64 for equivalent protection. The AP resistanceof the present invention (on an equivalent weight basis) is superior to300 Brinell hardness steel and equivalent or superior to 500 Brinellhardness steel. Moreover, the spall resistance of the composite of thepresent invention is greatly superior to that of homogeneous, highhardness steel plate.

Although the invention is described herein primarily as lightweightarmor plate with improved ballistic properties, the invention hasapplications in other fields. However, this invention is directed to arelatively thick composite material, i.e., one-fourth to 6 inches thick,and is not concerned with cladding to improve corrosion resistance,although such improvement may be obtained depending upon the selectionof the alloy composition.

What is claimed is:

Tablel Frontal Backing membser mer i iber Example Frontal membercomposition Backing member composition psi 1 6.6% Zn, 2.5% Mg, bal.aluminum... 5.5% Zn, 1.0% Mg, bal. aluminum... 88,000 57,000 2-.. do 086,000 57,000 3 do 5.5% Zn, 0.75% Mg bal. a1uminum 83,000 54, 000 4 dod0 0... 83 000 54,000

Table l Continued Frontal Backing member member 'I.S., I.S., ExampleFrontal member composition Backing member composition p.s.i. p.s.i.

7 ..d -.dO 81. 000 54,000 4.5% Zn, 4.2% Mg, bal. aluminum..- 5.0% Zn,0.5% Mg, bal. aluminum". 81,000 44,000 do .d0 80,000 58,000 10 70753.75% Zn, 2.0% Mg, bal. aluminu m 83,000 58,00

TABLE 11.-BALLISTIC PROPERTIES Nominal Backing composplate Merit rating*ite thick, thick, percent Cal. 30 Cal. 50 Cal. 50 mm. 20 mm. in. oftotal AP/lO" API45 AP/0 AP/30 FS Weight of 7039-164 (per unit area)required to protect *Merit rating thickness being between about 4 and 20percent of said total composite thickness and said backing member havinga tensile strength at least 2,000 p.s.i. less than the frontal platetensile strength, said tough aluminum alloy consisting essentially ofabout 4.0 to 6.0% zinc, from 0.5 to 3.5% magnesium, up to 3.0% copperand the balance aluminum and inconsequential X Weight of composite (perunit area) providing equivalent protection amounts of other elements.

2. The composite of claim 1 wherein the tough aluminum alloy contains0.5 to L5 percent magnesium.

3. The composite of claim 1 wherein the copper content of the frontalplate and backing plate is less than 0.2 percent.

4. The composite of claim 1 wherein the backing member thickness isbetween 5 and 12 percent of the total composite thickness.

5. The composite of claim 1 wherein the backing member thickness isbetween 5 and 12 percent of the total composite thickness.

6. The composite of claim 1 wherein the yield strength of the backingmember is at least 7,000 p.s.i. less than the frontal plate tensilestrength.

2. The composite of claim 1 wherein the tough aluminum alloy contains0.5 to 1.5 percent magnesium.
 3. The composite of claim 1 wherein thecopper content of the frontal plate and backing plate is less than 0.2percent.
 4. The composite of claim 1 wherein the backing memberthickness is between 5 and 12 percent of the total composite thickness.5. The composite of claim 1 wherein the backing member thickness isbetween 5 and 12 percent of the total composite thickness.
 6. Thecomposite of claim 1 wherein the yield strength of the backing member isat least 7,000 p.s.i. less than the frontal plate tensile strength.